Process and apparatus for producing continuous coatings



June 1961 c. J. DETTLING ETAL 2,987,413

PROCESS AND APPARATUS FOR PRODUCING CONTINUOUS COATINGS Filed March 25,1959 2 Sheets-Sheet l I Is Iii FIG. 2

INVENTORS CQNRAD J. DETTLING ROBERT E. HARTLINE w ATTORNEY lli? June 6,1961 c. J. DETTLlNG ETAL PROCESS AND APPARATUS FOR PRODUCING CONTINUOUSCOATINGS Filed March 23, 1959 2 Sheets-Sheet 2'IIIIIIIII/I/IIIIIIIIIIIIIIi INVENTORS CONRAD J. DET N6 8 ROBERT E. HARNE Bt-umL 1 620.: DM'K ATTORNEYS United States Patent 2,987,413 PROCESSAND APPARATUS FOR PRODUCING CONTINUOUS-COATIN GS Conrad I. Dettling andRobert E. Hartline, Reading, Ifa.,

asslgnors to The .Polymer Corporation, a corporation of PennsylvaniaFiled Mar. 23, 1959, Ser. No. 801,399 22 Claims. (Cl. 117-21) Thepresent invention relates to protective coatings and ma method andapparatus for producing such coatings on the surfacesof articles.

This application is .acontinuat'ion-in-p'art of copendmg applicationSerial No. 569,595, filed March 5, 1956, now abandoned, by Conrad I.Dettling and 'Robert E. Hartline.

The US. patent applications of Erwin Gemmer, Serial No. 427,481, filedMay 4, 1954,,and Serial No. 551,943, filed December 8, 1955 (nowabandoned), the U8. Patent No. 2,844,489, dated July 22, 1958, and theGerman Patent No. 933,091, disclose aprocess of applying coatings to avariety of objects by immersing the object in a fluidized bed ofpulverulent coating material that has a metling point lower than themelting or decomposition point of the material from which the particulararticles to be coated are made. The bed is fluidized by an upwardlymoving stream of gas passing therethrough which causes the bed toexpand. The article is heated while in contact with the fluidizedpulverulent material so that a certain amount of material adheres to andmelts on the surface of the article to form the coating. The methoddescribed in the Gemmer applications and patent is very useful-forforming coatings of materials which have superior resistance tochemicals and solvents or which are electrical insulators or which areresistant to mechanical abrasion or frictional wear. A variety of typesof coating materials may be used as described in the Gemmer applicationsand patent. The present invention is an improvement in the Gemmerprocess and apparatus.

In the practice of the aforementioned Gemmer invention, it is generallycontemplated that the articles to be coated are to be held or supportedin some positive manner by an article-holding means such as a pair ofpliers as the article is dipped into the fluidized bed and removedtherefrom. Although the Gemmer invention is not necessarily limited tothis feature, the Gemmer patents do not teach any other method for thesupport of the article as the coating is applied. Variousholding devicessuch as screw clamps may be used in addition to pliers. Small wireswhich may be soldered or welded 'to the surface of the article andremoved after coating, supporting hooks on which the article may behung, or other similar devices may also be employed.

It has been found that a very serious disadvantage arises from the useof such positive supporting means for the article to which the coatingis being applied. This disadvantage is that in every case there is adiscontinuity or interruption in the coating at the point or points onthe article engaged bythe holding device. This discontinuity in thecoating is likely to be particularly serious where thecorrosion-resistant properties of the coating are of primary importanceand where the coated article is to be exposed to a corrosiveenvironment. The coating discontinuity may also be a seriousdisadvantage where the purpose of the coating is for protection againstmechanical abrasion or for electrical insulation purposes. However, forthese purposes, it is often possible to arrange for the discontinuity toappear at a position on the article which does not impairits utility.

A further disadvantage of the Gemmer invention is that in certaininstances there is a coating thickness limitation which cannot beavoidedwithout repeated post heat- 2,987,413 Patented June 6, 1961 ingand reimmersionsteps. Continued repetition of this step is timeconsuming, and may be harmful to the initially applied coating.

Accordingly, it is another important object of the present invention toprovide a continuous coating of the above description, but having nocoating discontinuities occurring through the use of article-holdingdevices during the application of the coating material.

.It is a further object of this invention to provide a high qualitycoating of increased thickness with one immersion and no more thanonepost-heating step.

As previously noted the bed ofpulverulent coating'material in theopen-top container at first increases in volume when the air is turnedon until it reaches a fluidized state having what is considered amaximum volume consistent with avoiding an undue amount of geysering andentrainment of dust'from the container. In approaching the problems ofthe fluidized bed coating process of the Gemmer patent it has beenobserved that when the air flow is stopped or decreased substantiallythe bed at first shrinks rather rapidly (in the order of a few seconds)but thereafter contraction of the bed is very slow as long as the bed iskept quiescent and no positive pressure or vibration is applied thereto.

The bed in this quiescent flutfed state has very interesting properties.Clearly it is not fluidized for there is no air flow to maintainfluidization. However, it has some of the properties of the fluidizedbed in that an article can be inserted into it with little or noresistance and when immersed the powder will flow around the top of theirticle so as to cover and contact substantially all suraces.

However, in marked contrast to the fluidized bed, the fluffed powderwill support substantially unaided the entire weight of even ratherheavy objects such as metal objects. The same will be true of course forpowders that have volumes lower or densities greater than the fluflz'edpowder.

Of course all the various forms of powder are really a mixture of gasand pulverulent coating material, the different properties of thevarious beds being governed largely by the ratio of gas to coatingmaterial. In the fiuifed bed the ratio of gas-to-coating material issuch that the bed has substantially itsmaximum self-supporting volume,the particles of coating material being substantially immobile.

The degree of support provided bythe bed of pulverulent coating materialwill vary with the density. A completely fluidized bed will providerelatively little buoyant eflect, certainly not enough to support thearticle. On the other hand, a bed of substantially immobile particles ofcoating material at its maximum self-supporting volume will provideenough support to sustain the weight .of most articles.

Another interesting characteristic of the flufied bed is that since theparticles are substantially stationary, when heavy objects are immersedin the bed or other objects are immersed with applied force, there is atendency to compact slightly the powder beneath the article thereby tocushioniit and to keep it supported in the bed. This compacting effectis ordinarily not enough to prevent immersion but is enough to keep thearticle from descending all the way throughthe bed to the porous plate.

For the purpose of clarifying terminology, the following definitions ofterms and expressions used in the disclosure and claims are given. Theterm rarefied is applied to beds of pulverulent material that are eitherfluidized or fiuffed so as to permit the immersion of articles withfacility and without substantial resistance. Conversely, the termunrarefied refers to beds that have a lower volume and greater densityand therefore do not readily permit the immersion of articles therein.The term flufied has already been discussed "and implicitly defined andrefers to a bed of substantially immobile particles of the coating,material having substantially max imum self-supporting volume. Althoughthe method of producing the fluflt'ed bed has been previouslydescribedas involving, the initial fluidization .and subsequent deiluidization,it may be prepared by other procedures such as inverting a closedreceptacle containingthe powder, by vibration, or by other methods. v

' The term fluidized has been extensively discussed in the Gemmerapplications and patent. The corollary term unfluidized describes bedsin which, the particlesare substantially immobile and, therefore coversthe fluifed beds as well as the more densebeds that do not readilypermit the immersion of articles.

p In carrying out the present invention ,a bed of pulverulent coatingmaterial is formed comprising a mixture of a gas and the pulverulentcoating material. At least a portion of the mixture has a gas-to-coatingmaterial ratio in the range from that of a completely -fluidized densephase fluidized bed of the coating material to that of a bed ofsubstantially immobile particles of the coating material .atsubstantially its maximum self-supporting volume. The purpose of thisfirst named portion .of the fluidized bed is to make it easy to insertthe article into the body of powder and once inserted, if desired tomove the article around in the bed. Another characteristic of the bodyof powder is that it has at least a portion that is a mixture having agas-to-coating material ratio in the range from that of a bed ofsubstantially immobile particles of the coating material at its maximumself-supporting volume to that of unrarefied powder. One of the purposesof the second named portion of the bed is to provide support to thearticle. As previously mentioned and as more fully describedhereinafter, another very important characteristic of the second portionof the bed is that it has the capacity of forming a very thick'coatingonythe article by reason. of the particles being relatively stationaryand the absence of air flow which would tend to cool the article andcoating material and brush off particles that have only a very tenuousbond to the underlying particles in the layer.

It will be recognized that both portions encompass the fluffed powderthat has both the capacity to permit ready insertion into the bed andalso the ability to support the article.

v In a particular embodiment of the invention the first portion isfluidized either above the second portion or positioned laterally withrespect thereto, i.e., at substantially the same elevation.

The articlerto be coated is heated to a temperature above the melting'point of the coating material and is then immersed in the'bed ofmaterial so as to be completely surrounded thereby. During the period ofimmersion at least a portion of the surface of the article comes intocontact with the second named portion of the bed After the particles ofcoating material have been meltedand'coalesced on the surf-ace of thearticle and at least partly solidified the coated article is removedfrom the coating material.

One of the advantages of the present invention is in providing a coatingas described above in which no appreciable deformation of the coating iscaused by engagement of the coated article with the bottom or sides ofthe coating container. Another important advantage is in providing a wayof forming very thick coatings in a single heating and immersion cycle.Still other advantages will be apparent from the following descriptionand the accompanying drawings containing the following FIGURE 1 is 'asectional schematicview of a preferred form of apparatus for carryingout the present invention; wFIGURE 2 is an enlarged sectional view of amodi? fied formlof porous. plate containing nonporous portions shown inFIGURE 3 which may be advantageously used for carrying out the presentinvention;" I

FIGURE 3 is a top view of a porous plate containing nonporous portions;

FIGURE 4 represents an additional embodiment of this invention whereinthe container and porous plate are inclined at an angle of approximately45;

FIGURE 5 shows a typical magnetic pickup device for use according tothis invention; and

FIGURES 6A and 6B are directed to typical supporting surfaces shaped toconformrto the surfaces of a coated article, for use during apost-heating step.

Referring in more detail to FIGURE 1 of the drawings there is shownacontainer 10 which maybe constructed of a convenient structural materialsuch as steel, 'foi' instance, and which has an open top as indicated at12'. The container '10 is divided into an upper chamber 14 in which thepulverulent coating material is confined and a pressure chamber 16 by agas pervious partition 18. This partition, which should be pervious tothegas used but impervious to the particles .of coating material, maypreferably take the form of a porous ceramic plate, although othersimilar structuresmay be advantageously used. A porous plate structurewhich is preferred is composed of an Alundum refractory material formedof fused alumina grains bonded together with an aluminous glass at ahigh firing temperature as described in the Gemmer cases referred to.

The container 10 is provided with a gas inlet opening 20 which isadapted for connection through a shut-off valve 21 to a suitable sourceof gas under pressure in order to pressurize the, pressure chamber 16.The source of gas under pressure is not shown since it may consist ofany conventional source such as a steel bottle of precompressed gas or,'if air is to be used, a conventional air compressor and accumulationtank may be used. When air is to be used, it is also possible to attachan air blower or pump directly to the inlet connection 20. 1 In thepractice of the process ofthis invention, a quan; tity of very finelydivided coating material is placed in the upper chamber 14 of thecontainer 10 andgasunder pressure is admitted through the, connection 20into the pressure chamber 16. The gas from the lower chamber 16 passesthrough the gas-pervious partition 18 and flows upwardly through manyfinely divided streams .or in what might be characterized as a parallelupward flow from the entire upper surface of the partition 18 throughthe finely divided or pulverulent coating material. This upwardly movinggas causesrsome of the particles to be raised and separated from theothers in' what has been called a fluidization of the coating material.In this state, the coating material has the feeling and appearance ofaliquid although it is actually. simply a drymixture of gas and solidparticles. This .fiuidization of the coating material is such that solidobjects may be immersed within the coating material so as to becompletely surrounded thereby just as such anarticle might be-dippcdintoaliquid.

The portions of this apparatus and processdescribed immediately abovegenerally correspond tothe apparatus and process described'and claimedin the aforementioned Gemmer U.S. Patent-2,844,489; t

In accordance with one embodiment ofthe-present invention, rather thanholding the article which is to be coated as it is inserted into thefluidizing coating mate rial and removed therefrom, the article-afterheating, is simply inserted into .the fluidized material and releasedthereinr This may 'beaccomplished byreleasing the article over the opentop 12 of the container and permitting the article to drop into thefluidized. coating material in chamber 14. ---'I'he"article to be coatedacquires a continuous coating of the melted particles ofcoating/material. -Whcn coated lby thepresent process, e the a ti le s;,s di: t e ea in s spawns sists of a completely continuous inner portionwhich is directly adjacent and adherent to the outer surface of thearticle with successive superposed portions which are more porous, theoutermost portions of adherent coating material consisting of particleswhich have only melted slightly so as to have a degree of adherence tothe inner portions, but not enough to form a smooth and continuousappearance. Such a coating is entirely satisfactory for some purposes.However, for most purposes, it is desired to have a completely solid andsmooth appealing coating. This can be accomplished by applyingadditional heat to the article after it is removed from the fluidizedbath. This heat may be applied for a short period such as a few secondsby means of a gas flame, but preferably it is applied over a longerperiod such as by placing the coated parts in a suitable oven which isheated to a temperature slightly above the softening or meltingtemperature of the coating material.

In this post-heating step, it is necessary that the article be supportedon a surface which does not adhere to the softened coating and whichwill withstand the temperatures encountered. A suitable material forthis purpose has been found to be polytetrafluoroethylene. In order toprevent any substantial flow of the softened coating material away fromthe portions of the article engaged by the supporting surface due to theforce of the weight of the article, it is preferable to providesubstantial supporting surfaces which are shaped to conform tocorresponding surfaces of the coated article for engagement therewith.In this way, the force of the weight of the article is adequatelydistributed over the softened coating so that this force is insufficientat any one spot to cause a substantial flow and thinning of the coating.The problem is most simply solved, of course, if the coated article isprovided with a substantially large plane surface on which the articlecan be supported on a flat supporting surface.

FIGURES 6A and 6B of the drawings show supporting surfaces shaped tosubstantially conform to corresponding surfaces of the coated article.FIGURE 6A depicts a concave supporting surface 32 arranged to support acoated article 34 having a corresponding convex surface. In FIGURE 6B,the supporting surface 36 is in the form of a plane corresponding to theplane surface of the coated article 38.

It will be appreciated that this process permits the application of acoating which is absolutely continuous and completely surrounds andcovers all of the surfaces of the article which is to be coated.

A suitable device such as a wire basket 22 may be used to remove thecoated articles from the fluidized bed container, as described below.One method for avoiding damage of the newly formed coating and toconcurrently avoid any possibility of damage to the porous plate 18 isto provide a substantially unfiuidized or unrarefied layer of coatingmaterial in some portion, preferably near the bottom, of the chamber 14.The upper level of the unfiuidized or unrarefied portion of the coatingmaterial is indicated in FIGURE 1 of the drawings for instance at 24,while the upper level of the fluidized portion of the coating materialis indicated at 26.

It has been found that there are a number of practical methods forobtaining an unfluidized or unrarefied layer of the coating materialnear the bottom of the chamber 14. For instance, one excellent method issimply to shut off the supply of fluidizing gas to the chamber 16through the inlet 29. If the articles to be coated are dropped into thepulverulent coating material in chamber 14 immediately after thefluidizing gas is shut off, the upper portions of the coating materialremain sufiiciently rarefied so that the articles may drop through theupper layers so that they are completely submerged and surrounded withpulverulent coating material. The effect is somewhat similar to thatwhich is encountered when an vob- 76 iect is dropped into light,'fiufiy, dry new fallen snow. "In the use of this method, however, ithas been discovered that the lowermost portions of the pulverulentcoating material are not sufiiciently rarefied to permit the article tofall on through the bottom of the basket 22 or to the porous plate 18.Thus, the article rests and is supported entirely within the pulverulentcoating material with the result that a completely continuous anduninterrupted coating completely surrounding the article is formed.

The embodiment described immediately above, in which the flow offluidizing or rarefying gas is interrupted before the article is droppedinto the pulverulent coating material serves to illustrate that in thepractice of the present invention it is not absolutely essential that afluidizing gas must be used with a porous plate apparatus as shown inthe drawing as long as at least the upper portions of the pulverulentcoating material can be somehow rarefied. This rarefication might beaccomplished by other methods, for instance, such as by applying atemporary cover to the container and overturning the container and thenreturning it to an upright position. However, it has b en discoveredthat other methods for rarefication of the pulverulent coating materialare not generally as satisfactory as the fluidization with an upwardlymoving current of gas.

With the method of operation involving the interruption of the flow offiuidizing gas before the article is dropped into the container, it hasbeen discovered that unless the particles of coating material areextremely fine and fluffy, the depth of the pulverulent coating materialwhich remains sufficiently rarefied after the interruption of the gasflow is relatively limited and accordingly the largest articles whichthe coating container would normally be expected to accommodate will notsink into the pulverulent coating material to a depth which issufficient to provide for complete submergence and a coating over theentire article surface. One method for avoiding this difiiculty,however, is to lower the heated article into the pulverulent coatingmaterial while the fluidizing gas is flowing and then interrupting theflow of fluidizing gas to permit the formation of an unfluidized layerat the bottom of the chamber :14 before the article is released for complete support within the container.

It has also been discovered that another very satisfactory method is toestablish a very precise timing relationship between the instant atwhich the article is dropped through the upper opening 12 of thecontainer 10 into the chamber 14 and the instant at which the gas flowto the chamber 16 is interrupted. When the proper timing relationship isestablished, the upper portions of the rarefied coating material remainsuficiently rarefied to permit complete submersion of the article to becoated, but by the time the lower portions of the article approach thelower portions of the chamber 14, a sufiicient layer of unrarefiedcoating material has been formed near the bottom of the chamber 14 tosupport the article. It will be understood that the precise timingrelationship referred to above may involve the interruption of the gasflow to the chamber 16 either before or after or simultaneously with therelease of the article to be coated. The timing relationship will alsoobviously be related to the height from which the article is released.

It will be evident from the foregoing that in using the term immerse inconnection with particular types of powder beds, no particular timesequence of steps is implied unless the language so requires. Thus, thearticle may be inserted into a bed of one type and then the type of bedcan be changed and that would constitute immersion in a bed of thesecond type.

Another method for obtaining a relatively'unfiuidized or unrarefiedlayer of coating material near the bottom of chamber '14 is to simplyreduce the flow of gas through the porous plate 18 by reducing thepressure in the chamber 16. It has been discovered that with arelatively low ga flow, o y t pp po t n o e pu remlen coating materialare actually fluidized and the lower portions remain relatively dense.By proper adjustment of the gas pressure, therefore, a bottom layer ofpulverulent coating material which is sufliciently dense to'support thearticle being coated at the desired height can generally be obtained.

jAvariation of the low gas flow method described in the above'paragraphin which the upper level 24 of the unfluidized or unr'arefied bottomlayer of coating material may be rather definitely predetermined andcontrolled is by employement of relatively large, coarse, coatingmaterial particles for this bottom layer while employing the usual veryfine particles in the upper rarefied portions of the coating material.For instance, the particles in the lower unrarefied portions may have aparticle size in the order of from two to four times or more theparticle size of the rarefied portions. The preferred particle sizeranges for the practice of this invention are discussed more fullybelow. This method of using two distinctly different particles sizes isoperable partly because of the fact that the fluidization action of theupwardly moving stream of gas is effective to a greater degree on thefiner particles than on the larger particles. Accordingly, contrary towhat might be expected, the two different sizes of particles are notmixed to an appreciable degree unless the rate of gas flow is made to beextremely rapid compared to the rate required for fluidization only ofthe liner particles. Accordingly, with appropriate control of themaximum rate of flow of the fluidizing gas, the use of the coarseparticles of material to form the lower layer of unfluidized supportingmaterial provides excellent results.

Another method for providing an assurance of appropriate unfluidizedportions of the pulverulent coating material near the bottom of thechamber 14 may be accomplished by providing a regular pattern ofunfluidized spots or portions in the porous plate 18. A typical pattern,for instance, would be a geometrically regular polka dot pattern. Thisprocedure results in the maintenance of small mounds of unfluidizedpulverulent coating material on the upper surface of the porous plateimmediately above the nonporous portions thereof. This arrangement ismost effective when the article which is being coated is large enough tobe supported upon a number of these small unfluidized mounds. Thenonporous portions of the plate can be easily and simply provided byplacing small quantities of suitable sealing materials over such spotson the upper surface of the porous plate.

This modification of the structure of the porous plate 18 for carryingout this method is' illustrated in FIG- URES 2 and 3 in enlarged detailviews of only a portion of the porous plate 18. As described above,small nonporous portions of the plate are provided by means of spots ofsealing material indicated at 28 which at least adhere to the uppersurface of the porous plate and which may preferably penetrate to atleast a small depth into the surface of the porous plate. Variouswell-known substances may be used for this purpose, such as tar ortarlike compositions. As a result of the lack of porosity of these spots28 on the plate, mounds of unfluidized material accumulate above suchspots as indicated at 30 which provide the support for the articlesbeing coated as described above. 7

Another method-for providing an unfluidized portion of the bed forsupport purposes involves the tilting of a conventional container offluidized coating material as shown in FIGURE 4. When, for example, thecontainer 10 is tilted to an angle of approximately 45 from thevertical, an area of increased whirling is obtained at the shallow end40 of the upper chamber 14. The powder level, of course, remainsparallel to the floor. The powder in the deeper end 42 of the chamber 14is substantially less fluidized, and may be completely unfluidized,depending 'on the rate of gas flow through the porous plate 18.

1 In using this embodiment, the heated article is immersed m me shallowportion 40 of the chamber, thedesired amount-of coating is' permitted toadhere to the article,

and the article thereafter is moved into the deeper end 42 and released:Because of the powder in the deeper end 42' of thetilted container is ina less rarefiedconditiou, the lower reaches thereof provide a cushionpreventing the coated article from'touching the bottom or sides of thetank. After suflicient powdered material has adhered to the surface ofthe article, it is removed fromthe tank and subjected to a post-heatingstep if necessary.

All of the methods described above provide a cushion of substantiallyunfluidized pulverulent coating material for supporting the articlebeing coated above the air-pervious partition. In each of the processmodifications, the bed 'of pulverulent coating material must be rarefied(i.e., either fluidized or fiuffed) at some stage.

It has been found desirable in some instances to return the powderedcoated material in the container to a fluidized condition while thearticle to be coated is retained therein. In instances where the articlehas retained sufiicient heat to cause the coating material to fuse onthe surface of the article, the thin or poorly coated portions of thearticle may be again advantageously contacted with additional coatingmaterial by this subsequent fluidizing step.

The methods described above make possible the production of completelycontinuous coatings on articles by removing the necessity for a positivesupport other than the coating material itself. It has alsobeen foundthat the thickness of the coating obtained may be substantial lyincreased over the thickness of a similar coating ob tained according tothe process outlined in the aforementioned Gemmer patents. Theembodiment of this invention wherein the heated article is immersed in afluidized bed and the supply of fluidizing gas is thereafter interruptedto permit the fluidized bed to collapse about the heated article hasbeen found to be particularly effective in this regard. The articlebefore immersion is heated to a predetermined temperature above themelting point of the coating material, but below the rapid decompositiontemperature of the coating material. The latter temperature limitationcannot be exceeded without deleterious effects on the coating thusobtained. As the thickness of the coating depends upon the heat capacityof the article being coated, this upper temperature limit effectivelylimits the thickness of coating material deposited on the article duringimmersion thereof in a fluidized bed. In addition, it is believed thatattrition of the rapidly moving particles of the fluidized bed serves toremove the weakly bonded particles on the exterior of the coated articlewhile it remains in the fluidized bed. Regardless of the reason, it hasbeen found that coatings of from two to three times the thickness ofcoatings obtained from a fluidized bed may be obtained where thefluidized bed is employed to apply the initial coating followed by theretention of the thus coated article in the bed during collapse thereof,or in similar operations where the heated article is contacted withunfluidized, e.g., fluifed powder.

As to particle sizes of the-pulverulent coating material, it isparticularly important in the practice of the present invention thatcoating material particles of extremely small size be employed. While itis generally preferred that the particles should be in the size rangebetween about 0.001 and 0.012 inch, it has been found that the very bestresults are obtainable if the average particle sizes are kept below0.004. inch in maximum diameter and it is not particularly important ornecessary that the minimum sizeshould be above 0.001. Generally, thesmaller the average particle size of the pulverulent coating material,the higher the quality of the resulting coating. The particular aspectof quality which is referred to here is the smoothness andcontinuousness of the coating and the absence of minute discontinuitieswhich are sometimes referred to as pin holes.

The thickness of the coating which is produced is dependent upon anumber'of factors including (in addition -to the deg'reeof particlemovement): theparticle' size of the coating material, the time ofimmersion in thecoating' material, and the temperature and heat capacityof the article which is coated at the time of immersion in the coatingmaterial. Generally speaking, small powder particle sizes are conduciveto the production of thin coatings while large particle sizes areconducive to the production of thick coatings. Also, the greater thetime of immersion, the thicker the coating becomes up to the point wherethe heat of the article is substantially dissipated; The temperatureaffects the coating thickness in that the higher the temperature and thegreater the heat storage in the article which is coated, the thicker thecoating may be. Most importantly, however, all other factors remainingconstant, a much thicker coating is obtained where the particles areunfluidized.

In the practice of one embodiment of the present invention, the articlebeing coated will remain in the coating material until the formation ofthe coating is substantially complete and at least partially solidified.The time of immersion therefore is practically eliminated as a factor indetermining coating thickness for the article is left in the coatingmaterial until substantially all of the heat storage in the articleabove the melting temperature of the coating material has beendissipated. Accordingly, the accurate determination and control of thepreheating temperature of the article which is to be coated isparticularly important in this practice. Because of the wide variationsin heat storage characteristics of various different articles which maybe coated, it is very diflicult to specify preheat temperatures withoutdetermining suchtemperatures experimentally until coatings ofsatisfactory thicknesses are obtained.v However, it may be generallystated that when polyethylenes or polyamides are employed as the coatingmaterials, the articles to be coated may be preheated to temperatures asmuch as 300 F. above the melting temperatures of the coating materials.The thickness of coatings produced from such coating materials isgenerally in the approximate thickness range from .010 to .030 inch.

In order to obtain the thinnest possible coatings of uniform thickness,when the coated articles are removed from the fluidized pulverulentcoating material, the loosely adherent powder particles on the outersurface of the article may be removed by a rapidly moving stream of air,or by vibration or tumbling or other means prior to the post-heatingtreatment to form the smooth coating surface.

In the practice of the present invention, the removal of the coatedarticles from the fluidized coating material in chamber 14 may presentsomewhat of a problem. However,vthe basket 22 generally provides aconvenient solution to this problem. The basket 22 may be simply raisedfrom the chamber 14 through the top opening 12. The interstices of thebasket are small enough to prevent the coated articles from fallingthrough the bottom, while such openings permit the pulverulent coatingmaterial to fall through so as to separate the coated articles from thecoating material. The coated articles may then be quickly emptied fromthe basket 22 and the basket replaced in the position shown within thechamber 14. If preferred, the coated articles may be fished out of thepowder in chamber 14 by the use, for instance, of a mechanical rake orother similar mechanical devices other than the basket 22.

7 iii the articles coated are composed of magnetic materials, magneticpickup devices may be lowered into the pulverulent material to pick upthe coated articles by magnetic attraction. One such device is shown inFIG- DRE of the drawings. Referring to FIGURE 5 a magnetic article 50 isshown at the bottom of chamber '14 of the container of FIGURE 1. Thecontainer is provided with a pick-up device 52 comprising anelectromagnet 54 mounted at the lower end of a rod 56. Electricalconnections 58 are. provided between the electroma'gnet 54 and a powersource 60. The rod 56 is 10 mounted above the container 10 (mounting notshown) to raise and lower the device 52 with respect to the con tainer10.

It will be appreciated that the present invention readily lends itselfto conveyorization for mass production requirements. For instance, asuitable conveyor may be employed to convey the articles to be coatedthrough a preheating furnace and to drop the heated articles into thepulverulent coating material in chamber 14. Also, baskets smaller thanthe basket 22 which do not surround the entire interior of the chamber14 could be employed to sequentially pick up individual coated articlesfrom the chamber 14 after a timed interval during which the articlecools and the coating is formed. These baskets or other pick-up devicescould also be associated with a suitable conveyor so that the entireoperation would be on an automatic basis. Because of the fact that thisconveyorization could be carried out in a number of differentconventional ways, such a system is not illustrated in detail in thedrawings.

The following claims are intended to define the valid scope of thisinvention over the prior art and to cover all changes and modificationsfalling within the true spirit and valid scope of the invention.

We claim:

1. A process for the application of a continuous coating to an articlecomprising heating the article to a predetermined temperature, forming abed of coating ma terial comprising a mixture of a gas and a pulverulentcoating material, at least a portion of the mixture having agas-to-coating material ratio in the range of from that of a completelyfluidized dense phase fluidized bed of the coating material to that of abed of substantially immobile particles of the coating material at itsmaximum self-supporting volume, and at least a portion of the mixturehaving a gas-to coating material ratio in the range of from that of abed of substantially immobile particles of the coating material at itsmaximum self-supporting volume to that of an unrarefied powder, andimmersing the article in the bed so that at least a portion of thesurface of the article comes into contact with said second namedportion.

2; The process of claim 1 wherein the article is supported only by theforces provided by said second named portion of the bed during at leasta part of the immersion period.

3. The process of claim 1 wherein said portion of the surface of thearticle remains in contact with said second named portion of the beduntil the maximum amount of coating material has adhered thereto.

4. The process of claim 2 wherein said firs-t named portion of the bedof pulverulent coating material is fluidized within an open-toppedcontainer.

7 5. The process of claim 4 wherein the fluidized portionof the bed isin the upper part of the container and the second named portion of thebed is in the lower part of the container.

6. The process of claim 4 wherein the fluidized portion of the bed is atsubstantially the same elevation as the second portion of the bed andthe article is first immersed in the fluidized portion and thereaftermoved to the second named portion of the bed.

7. A process for the application of a continuous coating to an articlecomprising heating the article to a predetermined temperature above themelting point of the coating material, fluidizing a quantity ofpulverulent coating material to form a dense phase fluidized bed bymeans of an upwardly moving stream of gas within an opentoppedcontainer, thenimmersing the article in the fluidized bed and thereafterinterrupting the flow of gas without agitation of the coating materialto provide a bed of 'substantially immobile particles of the coatingmaterial "at its maximum self-supporting volume surrounding and V indirect contact with the surface of the article.

'8. 'A process for the application of a continuous coat ing to anarticle comprising heating the article to a predetermined temperatureabove the melting point of the coating material, fluidizing a quantityof pulverulent coating material to form a dense phase fluidized bed bymeans of an upwardly moving stream of gas within an open-toppedcontainer, interrupting the flow of gas without agitation of the coatingmaterial to provide a bed of substantially immobile particles of thecoating material at its maximum self-supporting volume, and thenimmersing the article in the substantially immobile bed.

9. A process for the application of a continuouscoating to an articlecomprising heating the article to a predetermined temperature above themelting point of the coating material, fluidizing the upper portion of aquantity of pulverulent coating material to form a dense phase fluidizedbed by means of an upwardly moving stream of gas within an open-toppedcontainer while maintaining lower portions of the pulverulent coatingunfluidized, and releasing and immersing the heated article in thefluidized coating material so that it is supported beneath the sur-'face of the fluidized material upon the unfluidized coating material.

10. A process for the application of a continuous coating to an articlecomprising heating the article to a predetermined temperature,fluidizing a quantity of pulverulent coating material to form a densephase fluidized bed by means of an upwardly moving current of gas whilesaid material is confined within an open-topped container, said materialhaving a melting point below said predetermined temperature,interrupting the flow of fluidized gas so that a bottom portion of thecoating material becomes unfluidized and releasing and immersing theheated article in the fluidized coating material for support upon theunfluidized portion.

11. A process for the application of 'a continuous coating to an articlecomprising heating the article to a predetermined temperature,fluidizing a quantity of pulverulent coating material to form a densephase fluidized bed by means of an upwardly moving current of gas whilesaid material is confined within an open-topped container, said materialhaving a melting temperature below said predetermined temperature,releasing the article for immersion in the coating material andinterrupting the flow of gas in a precise time relationship to providean unfluidized layer of pulverulent coating material for support of thearticle above the container bottom, the elevation of the article withrespect to the container at the time of release being'correlated withsaid precise timed relationship to prevent fall of the article throughthe entire bed.

12. A process for the application of a continuous coating to an articlecomprising heating the article to a predetermined temperature,fluidizing a quantity of pulverulent coating material to form a densephase fluidized bed by means of an upwardly moving current of gas whilesaid material is confined within an open-topped container, said materialhaving a melting point below said predetermined temperature, immersingthe heated article in the fluidized coating material and interruptingthe flow of fluidized gas so that the coating material becomesunfluidized to support the heated article. j V

. 13; A process for the application of a continuous coating to anarticle comprising heating the article to a predetermined temperature,fluidizing a quantity of pulverulent coating material to form a densephase fluidized bed by means of an upwardly moving current of gas whilesaid materialis confined within an open-topped container, said materialhaving a melting temperature below said'predetermined temperature, theflow of gas being precisely adiusted to fluidize the upper portions ofthe pulverulent coating material while maintaining a lowermost portionof the pulverulent coating material in an unfluidized condition,releasing and immersing the article in the fluidized coating materialfor support upon' the unfluidizedlayer while being completely surroundedby coating material.

;l4. --A- process for theapplicationofa continuous coatmaterial on asurface of a magnetizable article which comprises fluidizing the upperportion of a quantity oi material on a surface of anarticle whichcomprises ing to an article comprising heating the article to a prode--termined temperature, fluidizing a quantity of finepulverulent coatingmaterial to form a dense phase fluidized bed by means of an upwardlymoving current of gas while said material is confined within anopen-topped container, said material having a melting temperature belowsaid predetermined temperature, the container also being provided with abottom layer of relatively coarse particles of the same coating materialwhich are not fluidized, and releasing and immersing the heated articlein the fluidized coating material for support upon the unfluidized layerof coarse particles.

15. A process for the application of a continuous coating to an articlecomprising heating the article to a predetermined temperature above themelting temperature of the coating material, confining the coatingmaterial in a a porous bottom, fluidizing the pulverulent coating material to form a dense phase fluidized bed by forcing a current of gasthrough the porous container bottom while providing nonfluidizedportions of the pulverulent coating material immediately above theporous bottom by the provision of a regular pattern of nonporousportions in the porous bottom, and releasing and immersing the heatedarticle in the fluidized coating material for support upon thenonfluidized portions.

16. Apparatus for the application of coatings of dry, solid, pulverulentmaterial comprising a container having a substantial top opening and apartition enclosing a pressure chamber in the entire bottom areathereof, said partition including a regular pattern of porous portionswhich are pervious to gas under pressure but impervious to thepulverulent coating material under the force of gravity, the remainingportions of said partition being substantially impervious to gas underpressure, said container including a connection for introducing gasunder pressure to said pressure chamber.

17. Apparatus for the application of continuous coatings of dry, solid,pulverulent material comprising a container having a substantial topopening and a parti tion enclosing a pressure chamber in the entirebottom area thereof, said partition including a regular pattern ofporous and nonporous portions, said container including a connection forintroducing gas under pressure to said pressure chamber, and anopen-topped perforated container supported within said first-mentionedcontainer upon said partition.

18. The process of forming 'a continuous coating of material on asurface of an article, which comprises fluidizing the upper portion of aquantity of pulverulent coating material in a dense phase within anopen-topped container while maintaining lower portions thereofunfluidized, said coating materialcontaining particles therein whichmelt when heated, releasing and immersing the article in the fluidizedcoating material while heated to a temperature below the meltingtemperature of the article but at least as high as' the meltingtemperature of said particles and supporting the article upon theunfluidized coating material.

19. The process of forming a continuous coating of pulverulent coatingmaterial in a dense phase within an open-topped container whilemaintaining lower portions thereof unfluidized, said coating materialcontaining particles therein which melt when heated, releasing andimmersing the article in the fluidized coating material whileheated to atemperature below the melting temperature of the article but at least ashigh as the melting temperature of said particles and supporting thearticle upon the unfluidized coating material, and removin g'saidarticle from the coating material by the exertion of magnetic forcesthereon.

20;"The process of forming a continuous coating of fiuidizing the upperportion of a quantity of pulverulent coating material in a dense phasewithin an open-topped container while maintaining lower portions thereofunfiuidized, said coating material containing particles therein whichmelt when heated, releasing and immersing the article in the fluidizedcoating material While heated to a temperature below the meltingtemperature of the article but at least as high as the meltingtemperature of said particles and supporting the article upon theunfiuidizied coating material to form a coating thereon, removing thearticle from the coating material, supporting the article by engagementof a coated surface thereof with a supporting surface shaped to conformthereto, said supporting surface consisting of a nonadherenttemperature-resistant material, and reheating said article to completelyfuse the coating thereon while said article is so supported.

21. A process for the application of a continuous coating to an articlecomprising heating the article to a predetermined temperature,fluidizing a quantity of pulverulent coating material to form a densephase fluidized bed by means of an upwardly moving stream of gas withinan open-topped container, immersing the heated article in the fluidizedbed and thereafter returning the bed of coating material to anunfluidized state while the 14 article is immersed therein byinterrupting the upwardly moving stream of gas.

22. The process of claim 19 wherein a pulverulent coating material isrefluidized while the article remains immersed in the bed.

References Cited in the file of this patent UNITED STATES PATENTS466,441 Simonds Ian. 5, 1892 1,141,339 Hodgson June 1, 1915 1,534,846Fraser et al Apr. 21, 1925 2,059,983 Dent et a1 Nov. 3, 1936 2,622,043Roush Dec. 16, 1952 2,685,271 Essen Aug. 3, 1954 2,844,489 Gemmer July22, 1958 2,938,276 Doleman et a1. May 31, 1960 FOREIGN PATENTS 1,100,263France Sept. 19, 1955 OTHER REFERENCES Fraser et al.: Coal Age, vol. 29,No. 9, March 4, 1926, pages 325, 326, 327.

British Plastics, vol. 23, No. 255, August 1950, 117-21, pages 56-59.

1. A PROCESS FOR THE APPLICATION OF A CONTINUOUS COATING TO AN ARTICLECOMPRISING HEATING THE ARTICLE TO A PREDETERMINED TEMPERATURE, FORMING ABED OF COATING MATERIAL COMPRISING A MIXTURE OF A GAS AND A PULVERULENTCOATING MATERIAL, AT LEAST A PORTION OF THE MIXTURE HAVING AGAS-TO-COATING MATERIAL RATIO IN THE RANGE OF FROM THAT OF A COMPLETELYFLUIDIZED DENSE PHASE FLUIDIZED BED OF THE COATING MATERIAL TO THAT OF ABED OF SUBSTANTIALLY IMMOBILE PARTICLES OF THE COATING MATERIAL AT ITSMAXIMUM SELF-SUPPORTING VOLUME, AND AT LEAST A PORTION OF THE MIXTUREHAVING A GAS-TO-COATING MATERIAL RATIO IN THE RANGE OF FROM THAT OF ABED OF SUBSTANTIALLY IMMOBILE PARTICLES OF THE COATING MATERIAL AT ITSMAXIMUM SELF-SUPPORTING VOLUME TO THAT OF AN UNRAREFIED POWDER, ANDIMMERSING THE ARTICLE IN THE BED SO THAT AT LEAST A PORTION OF THESURFACE OF THE ARTICLE COMES INTO CONTACT WITH SAID SECOND NAMEDPORTION.